Republicans used to shut down the federal government with the intention (generally) of curbing public spending. President Trump’s GOP, in contrast, shuts down the federal government with the goal of constructing a continent-long wall of dubious utility that will require (by one estimate) three times the concrete poured in building the Hoover Dam.

The era of limited government is emphatically over in the only political party where it once had some appeal. The GOP’s nonnegotiable demand is now a monumental public works project. Somewhere in New Deal heaven, Harry L. Hopkins is smiling a mocking smile.

This is the strange case of a political metaphor slipping off the page and trying to break into reality. The images and symbols of political rhetoric can assume an importance beyond language. Ronald Reagan’s evocation of a “shining city on a hill” rooted his appeal in the American exceptionalism of our Pilgrim parents. John F. Kennedy spoke inspiringly of a torch of leadership passing between generations. Abraham Lincoln contended that a “house divided against itself cannot stand.”

But no one actually proposed getting the building permits for Reagan’s city, or forging Kennedy’s torch, or erecting Lincoln’s unstable house. Trump hopes to turn his metaphor into an object that could be seen from space. It is one measure of the madness of our political moment.

As a policy proposal, the wall is already a disaster. Candidate Trump pledged, again and again, that he would construct a physical wall along America’s 2,000-mile southern border, with Mexico footing the bill. Every element of that promise has been revealed as deceptive or impossible.

The Mexican government, predictably, has not been in the mood to fund the political obsession of a leader who accuses it of purposely exporting criminals to the United States. Even Trump administration officials seem to understand that Trump’s version of the wall is a pipe dream. “There are places,” outgoing chief of staff John F. Kelly conceded, “where hydrographically, geologically, a wall would not be realistic.” Sen. Lindsey O. Graham (R-S.C.) has described the wall as “a code word for better border security.”

By the standard of improved security, Trump’s plan is, at best, half-baked. A recent Government Accountability Office report warned of the possibility of massive waste because the administration’s border plans are so undeveloped. A proposal that may eventually cost $40 billion (in an estimate by MIT engineers) has been shaped more by the president’s political instincts than by serious study of alternatives. Agents in the field overwhelmingly request better technology and more personnel rather than longer and higher border barriers.

Trump has ascribed to the wall almost magical powers to fight murder, prevent gang activity and reduce opioid abuse. Never mind that violent crime rates among migrants are significantly lower than among the native-born. A barrier of the type that Trump embraces would not even address more routine problems. Smugglers are talented in finding ways above and below static barriers. They are effective in outsmarting officials at overwhelmed checkpoints. And one of the largest sources of illegal migration — outstayed visas — has nothing to do with a wall.

What might work better is an immigration system that allows seasonal workers to enter and leave the United States in an orderly fashion, and that treats women and children from Central America seeking asylum with dignity, thus allowing law enforcement to focus on real criminals and traffickers. To get that type of comprehensive immigration reform, I’d be willing to include some version of the wall in exchange. But without broader reform, a wall would likely end up as a useless ruin, a monument to vanity and bigotry.

Even as a political metaphor, the wall is badly lacking. It is the symbol of a political movement that has left liberty, inclusion and optimism behind it. Trumpism cultivates public fears to increase the role and power of the state. It locates national strength not in the character of a people but in the actions of an empowered leader. This is not even in the general category of conservatism.

Proposing a wall is really an argument that America can protect itself from the dangers of the world at its national boundaries. But this theory failed to contain the disorders of Europe and East Asia in the 1930s and 1940s. It failed to meet the global challenges and provocations of the Cold War. It failed to account for our vulnerability to terrorist threats emerging in Central Asia and the Middle East.

Putting our faith in a wall requires us to unlearn the bloodiest lessons of the last century. And to repeat them.